Rotary line trimmers are widely used for cutting grass, weeds and other plants in the upkeep of lawns and generally clearing vegetation. Line trimmers are typically characterized by a flexible nylon trimming line attached to a rotatable head. The head is attached to a motor shaft which is rotated at high speeds, typically by trigger operation of a small gasoline engine or electric motor. When the head is rotated, the rotating trimming line serves as a cutting blade to cut grass, weeds or other vegetation. The length of the cutting line extending from the head determines the overall diameter of the cutting path.
During use of line trimmers the cutting line often becomes worn and breaks off, eventually requiring replacement. Often the cutting line fails at the point where the cutting line extends from the head. One cause is the scoring of the cutting line from sharp or discontinuous surfaces on the head itself. Another cause is stress fractures caused by repeated bending of the cutting line during use. Still another cause is heating of the cutting line due to repeated bending, resulting in softening of the cutting line. Additionally, the heating of the cutting line can fuse the cutting line to the plastic surfaces of the head requiring replacement of the head or dismantling the head to remove the fused cutting line.
An additional problem in the prior art occurs after extended use of the line trimmer. The inventor has observed that after extended use, the line trimmer head collects a substantial amount of dirt, debris and plant matter both on the outside and inside of the trimming head. The problem is exacerbated by the presence of lubricating oils and greases typically used in line trimmers and which have a tendency to build up at the line trimming head. In most cases, the debris builds up to the point where it interferes with the mechanical operation of the trimming head.
An additional problem in the prior art occurs because the internal working components of the head are generally exposed to the external environment in which the line trimmer is operating. The external environment can include corrosive salt water, sand and salt residue typically found in coastal environments in which line trimmers are often used. Exposure to caustic environments can cause premature failure of the internal working components because of corrosion and salt residue buildup.
In prior art examples which include locking blades, the button used to bias the cutting line clamp becomes inoperable as its freedom of movement is blocked by a buildup of debris or corrosion. As the old soiled cutting line is removed from the head, one end must be pulled through the head. As the used cutting line is pulled through the head, dirt, excess vegetation and salt debris are pulled through the head as well. The debris is dislodged inside the head, it eventually blocks the movement of the locking blades requiring tedious cleaning of the internal parts or complete replacement of the entire head.
Still a further problem involves excessive wear of the plastic edges where the cutting line contacts the head. After prolonged use, the contact point(s) where the cutting line exits the line trimming head become jagged. The jagged edges wear the cutting line rapidly causing frequent failure and excess welding of the cutting line to the head. In most cases, the failures of the cutting line became so frequent that the entire head must be discarded.
Still another problem is friction caused by rotational imbalance in the line trimmer head. Various line trimmers in the prior art have not been adequately balanced about the axis of rotation of the line trimmer shaft. The imbalance results in vibration in the head which further results in additional friction at the exit portal for the cutting line. Additional friction results in premature wear of the head and premature failure of the cutting line.
Still a further problem in prior art trimming heads is insubstantial weight of the head. Line trimmers derive a significant amount of their effectiveness from the high speed of rotation of the trimming head. Maintaining the high speed during use requires a significant amount of energy. In use, cutting vegetation absorbs energy and has a tendency to slow the rotation of the head forcing the user to repeatedly back away from cutting to reestablish sufficient rotational speed. Maintaining sufficient rotational speed during use therefore requires larger engines and fuel tanks. However, most line trimmer heads of the prior art are made from injected molded plastic and therefore are of significantly light weight. The light weight of the prior art heads reduces the amount of rotational inertia needed to store energy. Some line trimmer heads of the prior art are provided with counterweights; however, the positioning of the counterweights has not been ideal or sufficient and has lead to other problems such as vibration.
Rotary line trimmer apparatus incorporating single length cutting lines having heads of various designs are known in the art.
U.S. Pat. No. 5,303,476 to Tuggle discloses a line head for a rotary line trimmer apparatus that securely holds a single cutting line. The apparatus includes a head with raised portions on its underside that define two channels. A bail including a clasp and a lip is recessed in the first channel. A length of cutting line is recessed in the second channel. The bail rotates between an open position facilitating insertion of the cutting line and a closed position designed to hold the cutting line during operation. While this apparatus secures a cutting line, it does not prevent premature cutting line breakage or minimize failure from accumulated debris.
U.S. Pat. No. 6,401,344 to Moore et al. discloses a head for a line trimming apparatus that includes a sliding cutting line clamp and generously curved cutting line contact points. The apparatus defines a pair of clamp arms that extend through a cutting line channel and secure a cutting line in the cutting line channel. The cutting line clamp is normally biased with a spring in the clamping position whereby the clamp arms press the cutting line against the cutting line channel. While the '344 patent discloses a method to more easily replace a trimming cutting line it does not address the issue of premature cutting line breakage due to heating or the issue of fusing of the cutting line to the internal plastic parts of the head.
U.S. Pat. No. 7,111,403 to Moore also discloses a head for a line trimming apparatus with a set metal inserts partially forming a cutting line channel. The inserts are sandwiched between upper and lower body sections of the head and provide a partial contact surface for the cutting line's lateral surfaces. A problem with the design disclosed in the '403 patent is that the cutting line tends to fuse to the internal plastic surfaces above and below the metal inserts. Also the minute gap that exists between inserts and the internal plastic surfaces creates a discontinuity in the cutting line channel which generates friction with the cutting line during use and results in the cutting line fusing itself to that gap and the internal plastic surfaces. The discontinuities also invite fouling and corrosion due to their tendency to scrape off dirt and debris from the cutting line inside the head.